1. Field of the Invention
The present invention generally relates to a computer graphics, and more particularly relates to a metaball drawing system using metaballs, which may be called “Blob” or “Blobby”. Also, the present invention relates to a metaball drawing method executed in such a metaball drawing system, and a memory medium storing a metaball drawing program for executing such a metaball drawing method.
2. Description of the Related Art
A concept of a metaball drawing method has been contributed by researchers of OSAKA UNIVERSITY in 1980s to draw and display a 2-dimensional image or a 3-dimensional image by smoothly curved lines (i.e., iso-density lines) or smoothly curved surfaces (i.e., iso-density surfaces). Also, a similar concept has been proposed in an article by James F. Blinn; entitled “A Generalization of Algebraic Surface Drawing”; ACM Transactions on Graphics (TOG), Volume 1, Issue 3, (July 1982), P. 235-256, and this drawing method is called “Blob” or “Blobby Model”.
A metaball is represented by a density distribution. For example, when two metaballs are defined on the screen of the monitor so as to partially overlap each other, the density distributions of the two metaballs are combined with each other. Then, a threshold or slice level is set with respect to the combined density distribution, and it is determined whether a pixel on the screen is included in the two metaballs. When the pixel is not included in the metaballs, a background luminance value (e.g. a black level) is set in the pixel concerned.
On the other hand, when the pixel is included in the metaballs, a density at the pixel concerned is calculated based on the combined density distribution. Then, it is determined whether the calculated density is above the threshold or slice level. When the calculated density is above the threshold or slice level, a given luminance value is set in the pixel concerned. When the calculated density is below the threshold or slice level, a setting of the background luminance value is given to the pixel concerned.
By processing all the pixels on the screen as stated above, an image is drawn based on the two metaballs. When the image is drawn as a 2-dimensional image, a contour of the image is defined by a closed iso-density line. Also, when the image is drawn as a 3-dimensional image, a contour of the image is defined by an iso-density surface. In either event, in the conventional metaball drawing method, the contour line or contour surface of the image is only drawn, and thus it is impossible to generate textures, patterns and so on in the interior area of the closed iso-density line or on the iso-density surface. Namely, the conventional metaball drawing method fails in drawing a complex and attractive image, as stated in detail hereinafter.